At http://www.dd-wrt.com/wiki/index.php/LaFonera_Hardware_Serial-Cable-Port you can read how to access the serial console on the Fonera.
But, the Raspberry Pi also has a serial port which operates at 3.3V. This means that you can connect the serial port of a Raspberry Pi and la Fonera without the need of a level shifter.
With a serial communication program like minicom it is now possible to access the Fonera.
starting minicom -b 9600 -D /dev/ttyAMA0
minicom : fonera booting